Separate helper for shutdown module check

Separate helper for shutdown module check

If a client was running during the upgrade from a sysvinit version
of the package, stopping it with the systemd unit file will not
necessarily have run the precheck script sufficiently recently to
fill in the KMOD variable with openafs, so that the shut-down client
would leave the kernel module loaded.  This causes great badness
when attempting to start the client again.

We do not want to reuse the precheck script during shutdown, as it
has conditionals on things like "not already running" which are not
appropriate there, so use a separate helper script for shutdown.
Always unload a module if it's present (unless loading is disabled),
to reduce the risk that subsequent starts will have trouble.